1. Field of the Invention
The present invention relates to deflection yokes and color cathode ray tubes with the deflection yokes.
2. Description of the Related Art
In the current color cathode ray tubes used as a display monitor such as windows, information is very often displayed in the peripheral regions of the screen. Therefore a technology enabling minute image display in such regions is being required.
Since the raster distortion is one of the important elements in determining the image quality in the peripheral regions of the screen, the standard for the raster distortion of the screen, which depends on the magnetic field distribution of the deflection yoke itself, has become very demanding.
In general, the magnetic field distribution at the screen side cone portion of a saddle shaped coil used as a horizontal deflection coil is designed to include a strong pincushion distortion in order to eliminate the raster distortion at the upper and lower edges of the screen. However, when it includes significant fifth-order pincushion distortion, an upper and lower high order raster distortion called gullwing emerges. Since a high order raster distortion such as the gullwing deteriorates the visual image quality drastically, it should be prevented.
In general, the vertical magnetic field distribution of a deflection yoke used in a color cathode ray tube for display monitoring has a barrel distortion entirely from the electron gun side to the screen side with respect to the self-convergence. Then, since the raster distortion at the right and left edges of the screen has a pincushion shape when such a barrel distortion is included, the distortion is eliminated by supplying a correction current from the circuit side of the display monitor toward the horizontal deflection coil. However, since the correction current in general has a wave form to correct a third-order pincushion distortion, when a raster distortion at the right and left edges of the screen includes a gullwing which is a high order distortion, the correction current can not completely eliminate the distortion. On the other hand, as mentioned above, since the gullwing drastically deteriorates the visual image quality, it should be prevented.
In order to meet such requirements, a method of reducing a high order raster distortion such as a gullwing at the upper and lower edges of the screen by forming a dent toward the central axis of the cathode ray tube at the center of the screen side flange portion of the horizontal deflection coil is proposed in U.S. Pat. No. 4,233,582. Another method of reducing the gullwing at the upper and lower edges of the screen by having the screen side flange portion of the horizontal deflection coil of a polygonal shape is advocated in U.S. Pat. No. 4,229,720. By analogy, these methods can be applied to a vertical deflection coil to reduce the gullwing at the right and left edges of the screen. Further, a method of reducing a high order raster distortion by forming a projection toward the electron gun side at the right and left edges of the screen side flange portion of a saddle shaped coil is proposed in Japanese Patent Application Laid Open No. 216738/1990.
However, in the method disclosed in U.S. Pat. No. 4,233,582, in the pressing process to provide a dent toward the central axis of the cathode ray tube at the center of the screen side flange portion of a horizontal deflection coil or a vertical deflection coil, there is a problem that it is highly likely that the insulating coating layer of a coil wire is damaged due to the excessive stretching of the coil wire in production. Further, if the dent is formed too deep, since the dent comes in contact with the funnel portion of the cathode ray tube when the deflection yoke is attached to the cathode ray tube, there is a problem in production or designing in that it is sometimes difficult to form a dent sufficient to remove a high order raster distortion such as the gullwing. Further, if a dent is formed too deep, since the dent comes in contact with the cone portion of the horizontal deflection coil when assembling the deflection yoke, there is a problem in production or designing in that it is sometimes difficult to form a dent sufficient to remove the gullwing. Further, in the method disclosed in U.S. Pat. No. 4,229,720, there is a problem in production in that coil wires are liable to be deformed and damaged at the apexes of the polygon-shaped screen side flange portion of the horizontal deflection coil or the vertical deflection coil.
In general, a ferrite core is used in a deflection yoke to strengthen the deflection magnetic field strength but the ferrite core also alleviates the magnetic field distortion formed by the deflection coil itself (hereinafter abbreviated ferrite core effect on the field distribution). Therefore even if the horizontal magnetic field distortion is controlled by the winding distribution of the deflection coil to minimize the deflection aberration, since the magnetic field distortion is alleviated by the ferrite core effect on the field distribution of the ferrite core, there is a problem that the correction sensitivity of the deflection aberration deteriorates to that extent.
In the method disclosed in Japanese Patent Application Laid Open No. 216738/1990, in the pressing process to provide a projection at the right and left edges of the screen side flange portion of the saddle shaped coil, there is a problem in that it is highly likely that the insulation coating layer of a coil wire is damaged due to the excessive stretching of the coil wire in production. Further, if the projection is formed too high, since the horizontal deflection coil, the vertical deflection coil and the ferrite core come in contact with each other when the deflection yoke is assembled, there is a problem in production or designing in that it is difficult to form a projection sufficient to remove a high order raster distortion.